"A Welsh teacher."
Isn't Welsh Cymraeg? How come it to be Gymraeg here?
Welsh, like other languages, has different masculine and feminine adjectives. Most other languages show this at the end of the adjective, eg blanco/blanca.
Welsh shows the difference between masculine and feminine adjectives by changes at the beginning of the word.
Athro = A (male) teacher ; Athro Cymraeg = A (male) Welsh teacher
Athrawes = A (female) teacher; Athrawes Gymraeg = A (female) Welsh teacher
The name for these changes is 'mutation'.
It is explained more fully in the unit 'May I'
Adjectives mutate after feminine nouns. Athrawes is a feminine noun. Cymraeg is an adjective here.
So, Cymraeg changes to Gymraeg => Athrawes Gymraeg.